Quick and accurate on-line measurements of gas flow rates at various pressure levels are required in a great variety of scientific, engineering and industrial fields for a large number of different purposes. For instance, in geological investigations, measuring the permeability of natural-gasbearing rocks is a key in assessing their value and importance. In a variety of other investigative and regulatory activities, gas flow measurements are needed to determine such parameters as air permeability of soils, gas permeabilities of potential waste repositories, radon-transmission properties of soils and rocks, and gas permeability of porous media at distinct levels of absolute gas pressures such as would be required in the petroleum industry. Still further uses of accurate gas flow devices include calibration of other, less accurate flowmeters, such as rotameters, and other types of indirect gas flow devices including thermal anemometers and turbine-type transducers. The usefulness of accurate gas flow devices, particularly those which can be employed over a wide range of gas pressures, cannot be understated.
It has been known in the prior art to use a wide variety of devices in order to perform the task of measuring gas flow rates for a number of different situations. Common devices used in gas flow measurements include the Stokes-law based float-type flowmeters, such as described in Gilmont et al, Instruments and Control Systems, Vol. 34, p. 2070 (1961), turbine-type flow transducers, as described in Dowdell, Flow: Its Measurement and Control in Science and Industry, Vol. 1, Part 2, page 687 (1974), thermal flow transducers, also described by Dowdell at page 549, and gasometers (see, e.g., Black et al, Methods of Soil Analysis, Part I, pp. 321-322, 1965). The float, turbine, and thermal devices measure momentum or thermal properties of the flow rather than directly measuring the flow rate. The accuracy of such indirect measuring devices is thus quite limited. With regard to gasometers, these devices cannot be used in on-line applications since they are "dead-end" devices. Measurements of true, steady-state gas flow cannot thus be obtained using these devices.
It has also been known in the past to use soapfilm devices to assess gas flow. The conventional soapfilm flowmeter (see, e.g., Kontes Scientific Glassware/Instruments General Catalog TG-60, p. 207, 1982) measures volume displacement with time and employs a flexible bladder as a soap solution reservoir. These devices are disadvantageous in that limits to low absolute values of gas pressure are imposed by the flexible bladder. Also known are reversing soapfilm flowmeters, such as described in Estes et al, Trans. AIME, Vol. 207, p. 338 (1956). These reversing soapfilm devices suffer from various drawbacks such as difficult soapfilm generation, manual timing functions subject to large operator error, and the need for periodic interruption and reversal of flow to keep the soapfilm in the device.
Soapfilm flow measuring devices are also known in the patent art. For example, U.S. Pat. No. 3,248,941 (McArthur) discloses a soapfilm flowmeter wherein a perforated movable bucket is employed in the formation of soapfilms. The mechanism that is employed for moving the bucket requires a mechanically sealed, rotatable shaft that passes through the body of the flowmeter, thus providing a potentially accuracy-degrading leakage path. In addition, this device is only applicable for low gas pressures, and a second embodiment is necessary for measurement of high pressure gases. Another device is disclosed in U.S. Pat. No. 4,691,577 (Lalin et al.) which employs soapfilms in gas measurements, but this complex flowmeter requires either an apertured or open-flared bottom tube as the flow-measuring channel. In addition, this device employs a flexible membrane that must be sealed against a movable object and a stationary object, and the seal thus provides a potential leakage path. Still other fluid flow devices are disclosed in U.S. Pat. No. 3,277,707 (Rodel), which requires an electrically conductive liquid film in addition to a mechanical pressure seal against a movable object, and No. 3,748,902 (Guild) in which the gas flow tube has sections having a variety of different diameters.
What is desired, therefore, and what has not yet been obtained in the prior art, is the development of a simple soapfilm flow meter device which employs a simple cylindrical non-apertured flow measurement tube and photoelectric or other accurate measuring means, which avoids the use of mechanical pressure seals against movable parts, and which can be used to obtain gas flow rates accurately, conveniently, and effectively over a wide range of gas pressures and a variety of different circumstances.